A vacuum gauge or evaporable getter consists in a deposit of a volatile and reactive material which is introduced inside the vacuum zone such that, once the zone has been sealed, the material is heated and evaporates, settling on the walls of the borosilicate tube and leaving a covering that will change colour due to oxidation if any vacuum loss takes place. They are usually used in vacuum tubes and the getter material used is barium. For big tubes, getters that are more complex are typically used, including aluminium, magnesium, calcium, sodium, strontium, caesium, and phosphorus.
The evaporable getter of the invention will be used in a vacuum tube as those employed as solar receptors. In general, those receptors comprise two concentric tubes between which a vacuum is generated. The internal tube, inside which the fluid being heated circulates, is metallic and the external tube is made of glass, usually borosilicate.
Between both tubes, a bellow shaped expansion compensator device is placed, such that it allows a relative movement between the absorbing tube and the glass tube, absorbing the strains that might be created by the difference existing between the expansion coefficients of both and thus ensuring the vacuum.
Besides the expansion compensator device, this type of tubes requires the installation of materials for detecting and suppressing the hydrogen molecules that might enter into the vacuum zone. These molecules are produced by the thermal degeneration that undergoes the oil used as heat-transfer fluid and that circulates inside the metallic tube, due to the high temperature that it reaches. These molecules end up passing through the metallic tube and entering into the vacuum zone, increasing looses, and consequently decreasing in a significant way the system effectiveness.
That is why there is always non evaporable-type getter alloys installed in the vacuum zone so that, in case there is any H2 molecule, it is uptaken by the surface of this material.
However, in addition to the non-evaporable getters, which solve the particles leakage, there are also installed evaporable getters, which detect if there have been any vacuum losses.
The development of the invention hereby disclosed describes a new geometry for supporting an evaporable getter.
Several background of this type of getters can be named, among others the patent ES 2241437 A1 of SCHOTT. This document discloses a clamp to retain the elements supporting the getters (or vacuum gauges). This clamp comprises a zone for supporting the evaporable element and the zone serving as a support in the receiving tube. Said zones count with certain resilience and are made with plates. Both zones are provided with pins for attaching them to the receiving tube and for supporting the tube. The pins of both zones are joined together through two flexural rigid edges.
This geometry is adapted to the type of receiving tube owned by the applicant SCHOTT, but would not be valid for other designs in which, for example, there might be a borosilicate tube, concentric with an absorbing tube, given that it would interfere with the last, besides requiring a completely hand-made assembly.
The patent US2004134484 also discloses another getter support, but in this case, it is formed by a channel (37) thereto are coupled pins (36) that secure it to the absorbing tube.
The patent CN201209968 also discloses an evaporable getter support that can be secured more easily to the absorbing tube but that also requires several pieces and welding for its manufacturing and positioning.
This is the reason why the object of the present invention is to provide a new geometry or a new design for supporting an evaporable getter, such that it can be adapted to all kinds of receiving tubes besides attaining a more automated assembly.